1. Field of the Invention
This invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material, utilizing a coloring reaction between a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto, which thermosensitive recording material has high preservability.
2. Discussion of Background
Conventionally, there is known a thermosensitive recording material, in which a thermosensitive coloring layer mainly comprising a thermosensitive coloring composition is provided on a support such as a sheet of paper and synthetic paper, or a plastic film. In such a recording material colored images are obtained by application of heat to the recording material using a thermal head, thermal pen, laser beam, stroboscopic lamp, and the like.
This type of thermosensitive recording material has wide-scale utilization, not only as a recording material for copying books and documents, but also as a recording material for use with printers for electronic calculators, facsimile machines, ticket vendors, and label-recorders because of the following advantages over other conventional recording materials:
(1) image recording can be speedily performed, using a comparatively simple device without complicated steps for development and image fixing; PA1 (2) the thermosensitive recording material can be produced and used without generating noise and causing environmental pollution; and PA1 (3) the manufacturing cost of the thermosensitive recording material is low.
The thermosensitive coloring composition for use in this type of thermosensitive recording material usually comprises a coloring agent and a color developer capable of inducing color formation in the coloring agent upon application of heat thereto. Conventionally, as such coloring agents, colorless or light-colored leuco dyes having, for example, lactone, lactam, or spiropyran rings, are employed; and as such color developers, organic acids and phenolic materials are employed. The thermosensitive recording material in which the aforementioned leuco dye and color developer are used in combination is widely used, since the color tone of the produced images is clear, the whiteness degree of the background of the thermosensitive recording material is high, and the produced images have excellent weathering resistance. The above-mentioned thermosensitive recording material, however, has the shortcoming that recorded images become easily discolored or fade away when the recorded images come into contact with oil and a plasticizer contained in a plastic film.
To eliminate the above shortcoming, it is known that an overcoat layer comprising a water-soluble polymer is formed on the thermosensitive coloring layer and that a backcoat layer comprising a water-soluble polymer is formed on the back side of the support so as to give plasticizer resistance to the thermosensitive recording material. However, the thermosensitive recording material comprising the overcoat layer and the backcoat layer comprising a water-soluble polymer also has the shortcoming. While the thermosensitive recording material in the form of a roll is stored under circumstances of high temperature and high humidity, the neighboring overcoat layer and the backcoat layer become sticky, namely, the so-called blocking problem occurs.
Several solutions to the above blocking problem have been proposed. For example, Japanese Laid-open Patent Application No. 61-57386 discloses a backcoat layer comprising alkali salts of styrene - maleic acid copolymer; Japanese Laid-open Patent Application No. 62-32081, a backcoat layer comprising colloidal silica; Japanese Laid-open Patent Application No. 63-230388, a backcoat layer comprising electroconductive titanium oxide. However, these thermosensitive recording materials do not have sufficient water resistance and barrier properties because an emulsion or a latex is employed for the backcoat layer of the thermosensitive recording material.